Issue #10990: Prevent tests from clobbering a set trace function.

Many tests simply didn't care if they unset a pre-existing trace function. This
made test coverage impossible. This patch fixes various tests to put back any
pre-existing trace function. It also introduces test.support.no_tracing as a
decorator which will temporarily unset the trace function for tests which
simply fail otherwise.

Thanks to Kristian Vlaardingerbroek for helping to find the cause of various
trace function unsets.

1 files changed, 10 insertions, 0 deletions

diff --git a/Lib/test/test_sys_settrace.py b/Lib/test/test_sys_settrace.py
index 43134e9..acb8e29 100644
--- a/Lib/test/test_sys_settrace.py
+++ b/Lib/test/test_sys_settrace.py
@@ -251,6 +251,7 @@ class TraceTestCase(unittest.TestCase):
     def setUp(self):
         self.using_gc = gc.isenabled()
         gc.disable()
+        self.addCleanup(sys.settrace, sys.gettrace())
 
     def tearDown(self):
         if self.using_gc:
@@ -389,6 +390,9 @@ class TraceTestCase(unittest.TestCase):
 
 
 class RaisingTraceFuncTestCase(unittest.TestCase):
+    def setUp(self):
+        self.addCleanup(sys.settrace, sys.gettrace())
+
     def trace(self, frame, event, arg):
         """A trace function that raises an exception in response to a
         specific trace event."""
@@ -688,6 +692,10 @@ def no_jump_without_trace_function():
 
 
 class JumpTestCase(unittest.TestCase):
+    def setUp(self):
+        self.addCleanup(sys.settrace, sys.gettrace())
+        sys.settrace(None)
+
     def compare_jump_output(self, expected, received):
         if received != expected:
             self.fail( "Outputs don't match:\n" +
@@ -739,6 +747,8 @@ class JumpTestCase(unittest.TestCase):
     def test_18_no_jump_to_non_integers(self):
         self.run_test(no_jump_to_non_integers)
     def test_19_no_jump_without_trace_function(self):
+        # Must set sys.settrace(None) in setUp(), else condition is not
+        # triggered.
         no_jump_without_trace_function()
 
     def test_20_large_function(self):